1) You can really mix a lot of different new colors out of these ones; skin tones, greens, browns, purple, orange, pink etc.
2) This type of color dries very quickly so you have to hurry up and cover the canvas in paint before it is too late, or you´re not going to be able to blend them well. When you only work with a few colors I have found it easier to paint rapidly and not to waste color from the palette.
3) From an economical point of view, I prefer to buy BIG but few buckets of paint which I know I use on a regular basis.
4) If I cannot mix a certain shade of lets say green, I can always fix it when I paint the following layers in oil.